This invention relates to a position detecting apparatus and, in particular, to the arrangement of loop coils of the position detecting apparatus.
An electromagnetic tablet together with a wireless position pointer has been known to be an effective input device to a computer system. The position pointer, in general, has a tuning circuit which resonates with a first electromagnetic wave from the electromagnetic tablet to generate a second electromagnetic wave. The tuning circuit typically has a LC circuit. The electromagnetic tablet has a plurality of electromagnetic wave generating and detecting coils. When a signal with a specified frequency is generated by a generating coil, current and voltage are induced within the tuning circuit of the position pointer. At this moment, the wave generating coil is disabled and the second electromagnetic wave is generated from the LC tuning circuit due to the electromagnetic effect of the induced current and voltage.
The second electromagnetic wave is detected by a detecting coil of the electromagnetic tablet and therefore a current and voltage are induced within the detecting coil. The induced voltage on the detecting coil is a function of the distance between the position pointer and the detecting coil. The plurality of generating coils are sequentially supplied with electric current to generate the first electromagnetic wave and the plurality of detecting coils are sequentially detected so that the position pointed by the position pointer is identified by the position of the detecting loop coil in which the highest voltage is induced.
The above describes the basic principle of the wireless position detecting apparatus. An in-depth description of such apparatus is set forth in U.S. Pat. No. 4,878,553, the disclosure of which is incorporated by reference.
The principle involved in the electromagnetic wave transmission of the position detecting apparatus mentioned above is the electromagnetic effect. As a result, when there exists any interference magnetic field within the vicinity of the detecting coils, malfunction of the position detecting can easily occur. For a less serious malfunction, accuracy of the position detecting apparatus is sacrificed. For a more serious malfunction, the position of the position pointer may not even be detected. Since the power source of the wireless position pointer is a battery or a passive type source, the output power is so low that its accuracy is easily influenced by the interference magnetic field.
There are two kinds of coil arrangement according to the conventional art. The first one may be called a single-direction looped coil arrangement as shown in FIG. 1 and the second one may be called a hi-direction looped coil arrangement as shown in FIG. 2. The figures show coils for position detection along a single axis. The tablet would normally also include a set of coils and associated circuitry for position detection along a perpendicular axis.
In FIG. 1, the position detecting apparatus consists of a plurality of coils L1, L2, . . . , each of which loops in a clockwise direction and defines an area and an inductance value. The area defined by each coil, in most cases, overlaps each other as shown to minimize the overall size of the electromagnetic tablet. One end terminal of each coil is coupled to the ground and the other end is respectively coupled to a transmission circuit 11 and a receiving circuit 12 through a first switch device SW1 and a second switch device SW2.
The operation of the apparatus shown in FIG. 1 is as follows. At first, first switch SW1 is switched to the A1 terminal and second switch SW2 is switched to the B0 terminal. At this time, transmission circuit 11 supplies a current to first coil L1 which generates an electromagnetic wave to the position pointer 13. After a later time on which the LC circuit in position pointer 13 generates a current and voltage, first switch SW1 is switched to the A0 terminal and the second switch SW2 is switched to B1, and first coil L1 acts as detecting loop coil at this moment. The electromagnetic wave generated from position pointer 13 is then detected by first coil L1 and through second switch SW2 the induced current and voltage is then received by receiving circuit 12. Through the switching of the terminals A0, A1, . . . and B0, B1, . . . , the highest voltage induced may be found and the position pointed by pointer 13 may be determined.
One of the drawbacks of the above mentioned coil arrangement is the vulnerability to electromagnetic noise. One may only screen the noise by a filter after the induced signal is amplified by the receiving circuit 12, but this will definitely affect the true signal and degrade the accurate position measurement.
In FIG. 2, the position detecting apparatus consists of a plurality of coil assemblies L1, L2, . . . However, each coil assembly has two coils with oppositely looped direction. For example, coil assembly L1 has one clockwise looped coil L1F and one counterclockwise looped coil L1R. Coil L1F is spaced apart from coil L1R by an amount of W/2, where W is the overall width of the electromagnetic tablet. The dashed line 20 is the center line of the tablet. The function block 21 has the same function as the transmission and receiving circuit 14 of FIG. 1.
The operation of the position detecting apparatus of FIG. 2 is generally the same as that of the apparatus of FIG. 1. However, since coils L1F and L1R are used together to generate the electromagnetic wave to the position pointer 23, in order to accurately determine the position pointed, a polarity determination has to be made for the effect of the counterclockwise coil L1R. That is, the use of the second coil introduces an ambiguity since the determination of the highest voltage does not distinguish which of the two coils is closest to the pointer. The polarity of the detected signal must be determined to resolve this ambiguity.
The bi-direction looped coil as shown in FIG. 2 is more immune to noise than the coil arrangement shown in FIG. 1. As long as the interference source is not concentrated in the area of the coil pointed by the pointer 23, the interfering effect imposed upon the clockwise coil will be offset by the interfering effect imposed upon the counterclockwise coil. As a result, interference is minimized. However, since the clockwise and counterclockwise coil are spaced by w/2, if the interference source is concentrated on the vicinity of the pointer 23, a substantial interference is felt. Thus, coil arrangement of FIG. 2 is effective to immunize from the noise only when the noise is distributed evenly over the entire area of the tablet or the interfering source is distant from the tablet.